Substrate support for use in a hot filament chemical vapor deposition chamber

ABSTRACT

A carbon deposition chamber is provided with several advantages. The substrate and the heating filaments are cooled to a temperature to prevent carbonization by permitting a cooling fluid to be passed through tubing connected to these elements in a heat sink like manner. The substrate is permitted to rotate back-and-forth to permit more even deposition of carbon films onto the substrate. The heating filaments are permitted to expand and contract without breakage by permitting the electrode attached to one end of the filaments to move freely as the filaments change in length. The gas mixture used within the deposition process is expressed from tubing through three zones, which are each individually determined with needle valves.

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional application for a patent is a continuation of theprovisional application for a patent entitled “HOT FILAMENT CHEMICALVAPOR DEPOSITION CHAMBER AND METHOD OF USE” filed on Aug. 30, 1999 bythe inventor, Zhidan L. Tolt and granted U.S. Ser. No. 60/151,420. Thisnon-provisional application relates to two non-provisional applicationsfiled Feb. 23, 2000 by inventor, Zhidan L. Tolt, entitled A GasDispersion Apparatus for Use in a Hot Filament Chemical Vapor DepositionChamber, Ser. No. 09/511,572, and A Heating Element for Use in a HotFilament Chemical Vapor Deposition Chamber, Ser. No. 09/511,021.

TECHNICAL FIELD

This invention relates to the chemical vapor deposition of diamond orcarbon films, and more particularly, to an apparatus and a method of usein such deposition.

BACKGROUND INFORMATION

One class of methods developed in recent years for carbon depositionconsists of the chemical vapor deposition (“CVD”) methods. For a generalsummary of various deposition methods including CVD methods, referenceis made to Chemical & Engineering News, 67(20), 24-39 (May 15, 1989),incorporated herein by reference.

In the CVD methods, a mixture of hydrogen and a hydrocarbon gas such asmethane is introduced into a chamber and is then heated or thermallyactivated. Some of the hydrogen gas is disassociated into atomichydrogen which reacts with the hydrocarbon to form growth species. Thesespecies deposit on the substrate in the form of a carbon film when theycome into contact with a cooler substrate. This process is schematicallyillustrated in FIG. 1.

One of the many of the CVD coating methods, hereinafter referred to as“filament” methods, employ one or more resistance heating unitsincluding heated wires or filaments, typically at temperatures of atleast 2000° C., to provide the high activation temperatures at whichthese disassociations take place. This method is known in the art as hotfilament assisted chemical vapor deposition (“HFCVD”).

Many reaction and mass transport processes occur on the substratesurface. The substrate temperature is therefore crucial to optimizegrowth of the film.

The integrity of the resistance heating filaments is critical inachieving and maintaining uniform temperature during the deposition. Ifa heating filament sags, the temperature in the vicinity of the heatingelement differs, thereby creating thermal and species nonuniformity onthe substrate. If filaments break frequently, maintenance is needed moreoften. The condition and position of heating elements is therefore acritical factor for reactor operation. Resistance heating filaments aredelicate and are easily broken during operation of the reactor. Theyexpand when heated from room temperature to an operating temperature of2000° C. and are thus subject to thermally-induced stresses that canbreak the filaments. Also, during carbon deposition, resistance heatingfilaments can react with carbon-bearing gases contacting the filamentsand form a carbide, further lengthening and embrittling the filaments.

Horizontal filaments which are typically used in HFCVDs present a numberof problems. The filaments severely deform and sag after they are firstheated up so that uniform and repeatable film deposition becomeimpracticable. As a result, intense maintenance work becomes necessaryand makes it impractical to use most conventional HFCVD reactors formanufacturing purposes.

Because of tremendous thermal gradients present in a hot filamentreactor, a large ceramic substrate plate breaks easily. For large areadeposition, a uniform gas distribution is also necessary to achieveuniform deposition. It is thus important to be able to control thetemperature of the substrate. Furthermore, graphitic carbon onnon-substrate (i.e., the elements of the reactor) accumulates in HFCVDreactors. This accumulation requires frequent removal resulting inhigher maintenance costs.

What is needed, therefore, is a device that can maintain a substantiallyconstant force on the filaments to eliminate sagging of the filaments.Preferably, such a device would minimize maintenance and equipmentset-up between cycles of operation. What is also needed is a device tocontrol the gas flow. Preferably, such a device would also control thedeposition of the carbon on the substrate and control the temperature ofthe substrate to prevent breakage of the substrate. Furthermore, apreferred device would also result in reduced maintenance costs byminimizing carbon accumulation.

SUMMARY OF THE INVENTION

The previously mentioned needs are fulfilled with the present invention.Accordingly, there is provided, in a first form, a disclosed CVD reactorwhich substantially reduces or eliminates the disadvantages andshortcomings associated with the prior art techniques.

To prevent filament sagging, the filaments in the present invention arearranged in the reactor vertically instead of the typical horizontalconfiguration. The filament assembly is configured such that the lowerend of each filament can freely contract or extend vertically whileconstrained in all other directions.

The reactant gas is introduced into the reactor chamber through a gasdispersion system. The gas dispersion system, mounted within the reactorchamber, has a configuration to introduce the gas into three separatefeeding zones. Each feeding zone has an independent control of the gasfeeding rate. The extent of radial gas flow in the reactor, therefore,is controlled. Thus, the typical nonuniform distribution of gas isreduced by utilizing a three-zone feed gas distribution so that auniform deposit of material can be achieved over the entire surface ofthe substrate.

A substrate support is mounted within the reactor chamber. During theoperation of the reactor, the substrate support continually rotates 180degrees back and forth. This constant rotation also helps ensure auniform distribution of the carbon onto the surface of the substrate.

To prevent substrate breakage, there is also a feature to preheat thesubstrate before the actual deposition process is started. Thispreheating reduces the thermal gradient created by the filaments. Thereis also provided a means to cool the substrate holder when the reactoris fully operational. This cooling mechanism allows the user toindependently control the temperature of the substrate.

Furthermore, the filament support structure and other relatively hotsurfaces inside the reactor are water-cooled so that little, if any,graphitic carbon can accumulate on these surfaces. Thus, maintenancecosts due to this accumulation are greatly reduced.

These and other features, and advantages, will be more clearlyunderstood from the following detailed description taken in conjunctionwith the accompanying drawings. It is important to note the drawings arenot intended to represent the only form of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view showing the chemical reactions and processesof a typical hot filament vapor deposition process;

FIG. 2 is a side-sectional view of the chamber illustrating oneembodiment of the present invention;

FIG. 3 is a cross-sectional view of the chamber illustrating thefilament configuration of one embodiment of the present invention;

FIG. 4 is another cross-sectional view of the chamber illustrating thegas dispersion configuration of one embodiment of the present invention;

FIG. 5 is a detail plan view illustrating the configuration of asubstrate cooling system of one embodiment of the present invention;

FIG. 6 is a schematic view of the operational aspects of the gasdispersion system illustrated in FIG. 4; and

FIG. 7 is a detail plan view illustrating the configuration of asubstrate support cooling system of one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The principles of the present invention and their advantages are bestunderstood by referring to the illustrated embodiment depicted in FIGS.2-7 of the drawings, in which like numbers designate like parts. In thefollowing description, well-known elements are presented withoutdetailed description in order not to obscure the present invention inunnecessary detail. For the most part, details unnecessary to obtain acomplete understanding of the present invention have been omittedinasmuch as such details are within the skills of persons of ordinaryskill in the relevant art. Details regarding control circuitry ormechanisms used to control the rotation of the various elementsdescribed herein are omitted, as such control circuits are within theskills of persons of ordinary skill in the relevant art.

Referring now to FIG. 2, there is depicted a side view of reactionchamber 200 showing the interior features of reaction chamber 200 of theHFCVD reactor according to the present invention. Chamber 200 is thehousing where the chemical reactions and depositions occur. All of thefeatures disclosed in the present invention are enclosed in reactionchamber 200 which is air-tight and thus capable of being maintained atreduced pressure and is fitted with a suitable gas inlet and an exhaustport (not shown). All portions of the apparatus which are present in thereaction chamber 200 are constructed of suitable heat-resistantmaterials, as necessary to withstand high temperatures. Stainless steelis an illustrative heat-resistant material suitable for this purpose.

FIG. 2 is a simplified sectional view of the HFCVD reactor 200 accordingto the present invention. In the following description of the invention,the reactor 200 is oriented horizontally and gas flow is from right toleft. However, this does not imply that the invention is limited to thisarrangement of flow. Vertical upward flow, and in some cases verticaldownward flow, are included as embodiments of the present invention.

The key elements of the improved HFCVD reactor 200 comprise a multi-zonegas dispersion system 202 (see FIG. 4), a filament array 206 (see FIG.3), a rotating substrate holder 208, a substrate cooling system 212, anda substrate heater 210. Gas dispersion system 202 and substrate holder208 are oriented within the reactor so that their surfaces areperpendicular to the axis of the gas flow through the reaction zone. Thesubstrate 216 to be coated is supported by substrate holder 208 whichrests on substrate heater 210. Under substrate heater 210 is substratecooler 212 which is mounted to substrate holder base 214. Substrateheater 210 is provided with a lead (not shown) to which an electricalheating current is conducted from a suitable external power source (notshown). Substrate holder 208 is also provided with a thermocouple (notshown) to measure substrate temperature and a connecting electrical lead(not shown) through which the thermocouple output may be transmitted toan external indicator or read-out (not shown).

Substrate holder base 214 is connected to turning axle 215, which ishollow to permit the passage of fluid conduits 251 and 252, which willpass the cooling fluid for substrate cooler 212, along with theelectrical lead (not shown) for the thermal couple (not shown) and thesubstrate heater 210. Motor 253 turns axle 215, resulting in the turningof the substrate 208. This is performed in a back-and-forth motion wherethe substrate is rotated 180° clockwise and then returned 180°counterclockwise. Note that a lateral displacement of the substrateholder is also permitted.

Chamber 200 also includes view ports 220 and 221 for looking at theinternal contents of the chamber 200. Additionally, chamber 200 includesthe hinged lid 218, which can be opened to access the internalcomponents of chamber 200,

Label 204 indicates that the gas dispersion system 202 can be movedback-and-forth to be closer or farther away from the substrate 208. Theremainder of the components noted in FIG. 2 are discussed in furtherdetail with respect to FIGS. 3-7.

FIG. 3 illustrates the vertical array 206 of heating filaments 302.Filaments 302 are made of a material that heats upon passing anelectrical current through it. Means for applying an electrical currentare connected to electrodes 304 and 306. Illustrative materials aremetallic tungsten, tantalum, molybdenum, and rhenium, with tungstenbeing preferred. Although the embodiment in FIG. 3 shows preferredvertical filaments 302, several features of the embodiments of thisinvention are also applicable to a reactor which utilizes horizontalfilaments instead of or in addition to the vertical filaments.

Resistance heating filaments 302 are of approximately the same length asthe other filaments and are attached to two electrodes (filament bars)304 and 306 to form the array of filaments 206. The top electrode 304has its position fixed for reactor operation.

Electrode 306 is also connected to filaments 302. However, as moreclearly illustrated in FIG. 2, electrode 306 is permitted to slidebetween positioning posts 260 and 261, which are affixed to the chamber200 wall by base 361. Base 360 affixes electrode 304 to the chamberwallalso. Positioning posts 260 and 261 are made of a material, such as aceramic, which is not only insulative, but also has a surface thatpermits the slidable movement of electrode 306 in a vertical directionas the filaments 302 lengthen from heat and carbonization, or shorten asthey are cooled. This alleviates the longitudinal stress upon thefilaments 302 as the temperature in the chamber 200 changes.

Cooling of the electrodes 304 and 306 is provided by the cooling fluidpassing through conduits 352-354, which pass the cooling fluid fromflexible tubing tubings 350 and 351, which receive and send the coolingfluid to and from a source external to the chamber 200.

During operation, filaments grow from carburization and from thermalexpansion. The gravity force on the movable electrode maintains thefilaments taut. The movable electrode of the array is also preferably sodesigned that it rigidly clamps its filaments' ends, thereby preventingsubstantial vibration of the filaments at the spring end. For verticalfilaments, it is preferable to provide guide tracks on each end of themovable electrode to prevent unwanted spiraling and bouncing of themovable electrode and filaments that could be caused by the flow of hotgases around the filaments and movable electrode.

A conventional HFCVD reactor design utilizes a flat rectangular orcircular substrate holder over which a small substrate is placed for CVDcoating. The gas flow adjacent to the substrate surface is thereforegenerally radial outward from a stagnation point, resulting innon-uniform deposition.

FIG. 4 illustrates the gas dispersion system 202 whereby three separatefeedlines 611-613 supply the reaction mixture of gases in three separatezones further depicted in FIG. 6. These three zones are labeled as 615,616 and 617, which zones are concentric. Each of the pipes associatedwith each zone are perforated for expulsion of the gas towards thesubstrate in their respective zones.

Gas is fed to a mixing chamber 604 through a valve 603 from a gas sourceor reservoir 601-602. The mixed gas is then passed through needle valves605-607 to the respective gas feedlines 611-613 to zones 615-617,respectively, which lie within the chamber. The barometric pressure canbe measured at gauge 615 through open/close valves 608-610 individuallyfor each gas feedline 611-613. The needle valves fine control the flowrate of each line to the different zones of the reactor to optimize thegas distribution. The shape of the gas dispersion elements or perforatedtubes can be different for different substrate geometries. As describedabove, the distance between the perforated gas line assembly and thefilaments is also adjustable.

FIGS. 5 and 7 illustrate front and side views of cooling tubingconfigurations for the substrate cooler 212, whereby the cooling fluidis passed to either one of these types of configurations for cooling thesubstrate.

In operation, the reaction chamber 200 is maintained at a pressure up toabout 760 torr, typically on the order of 10 torr. A mixture of hydrogenand a hydrocarbon, most often methane is passed into the chamber 200 anda current is passed through the electrodes 304, 306 and filaments 302 toheat the filaments 302 to a temperature of at least about 2000° C.

The heat sink(s) is maintained at a distance from the substrate andwater passage through the tubing associated therewith is maintained at arate to provide a substrate temperature in the range of about 800-1000°C., depending on the desired film structure and growth rate.

During the CVD operation, filaments 302 undergo thermal expansion andexpansion due to carburization. By reason of their prestressedcondition, however, such expansion merely causes them to lengthen. Thus,they are not prone to distort in other directions or break. Using thiscombination of elements, the filaments will have a longer life span.

Although the invention has been described with reference to specificembodiments, these descriptions are not meant to be construed in alimiting sense. Various modifications of the disclosed embodiments, aswell as alternative embodiments of the invention will become apparent topersons skilled in the art upon reference to the description of theinvention. It is therefore, contemplated that the claims will cover anysuch modifications or embodiments that fall within the true scope of theinvention.

What is claimed:
 1. A hot filament CVD reactor comprising: a hotfilament array; a substrate holder for holding a substrate substantiallyparallel to said hot filament array and within a gas path; a substrateheater connected to said substrate holder downstream from said substratewithin said gas path; a substrate cooler functionally connected to saidsubstrate heater and said substrate holder and positioned downstream ofsaid substrate within said gas path; a rotating axle functionallyconnected to said substrate cooler, said substrate heater and saidsubstrate holder, said axle slidably connected to a wall of said reactorfor lateral movement of said substrate cooler, said substrate heater andsaid substrate holder; and a moving apparatus connected to said axle andadapted to rotate said substrate cooler, said substrate heater and saidsubstrate holder continually 180 degrees back and forth during a coatingprocess to ensure a uniform distribution of a CVD coating on the surfaceof the substrate.
 2. A method of supporting a substrate in a reactor fordepositing a film on the substrate, the method comprising the steps of:providing a substrate support having a substrate holder maintaining asubstrate, a substrate heater, an independent substrate cooler and anaxle, said substrate support being laterally and rotationally moveablein said reactor; positioning said substrate substantially parallel to ahot filament array; pre-heating said substrate via said substrateheater; emitting a gas stream through said hot filament array andsubstantially perpendicular to said substrate wherein said substrateheater and said substrate cooler are positioned downstream of saidsubstrate within said gas stream to form a CVD coating on the surface ofthe substrate; cooling said substrate via said independent substratecooler; and rotating continually said substrate support 180 degrees backand forth during the coating process to ensure a uniform distribution ofThe CVD coating on the surface of the substrate.
 3. The method of claim2 wherein said substrate heater is electrically energized.
 4. The methodof claim 2 wherein said substrate cooler comprises a fluid.
 5. Themethod of claim 2 wherein said substrate heater is electricallyenergized and said substrate cooler comprises a fluid.